When Qin shihuang conquored china, he put in place a legalistic system where bueracrats ruled china and brought the former noble families to his capital. There he kept them under house arrest and basically paid for whatever expences they wanted to keep living as nobles. Why? It's morbid, but logically, it would make sence in both an attempt to save money and to prevent possible nobles from rising up against him in the future to just kill them. Whats more, considering the amount of bloodshed he used to kill enemy armies and sack cities, it would make sense to throw the nobles he conquered onto the metaphorical firepit of corpses he created. So why didn't he?
It's morbid, but logically, it would make sence in both an attempt to save money and to prevent possible nobles from rising up against him in the future to just kill them
Actually, it's the exact opposite. Keeping "guests" (aka hostages) as a form of peace-keeping is a long-standing tradition in various countries in various moments in history. Adam Kosto's book Hostages in the Middle Ages explores this phenomenon in Europe.
If your relative is captured by a hostile enemy, the likelihood of you laying siege to said enemy is far reduced, for fear your relative will be killed. When Qin Shihuang gathered the former noble families, he didn't gather every single member of those families. Those left alive and free would be less inclined to attack him, especially if their relatively were well cared for. Contrast this to the notion that your family member is being held hostage and viciously tortured. In this case, you'll stop at nothing to free them. A well-fed, happy prisoner is far less likely to induce a rebellion.